1. Field of the Invention
This invention relates to the chemical modification of an aluminum oxide supported rhodium catalyst that forms the basis for an improved automotive catalytic converter useful in environmental protection. More specifically, this chemical modification involves the use of a silation agent to remove surface hydroxyl groups from the aluminum oxide so that the conversion of active rhodium Rh.sub.x.sup.(0) to oxidized rhodium species Rh.sup.(I) by reduction of the active hydroxyl groups cannot substantially occur.
2. Brief Description of the Prior Art Automotive catalysts for controlling automotive exhaust emissions are comprised of the noble metals platinum (Pt), palladium (Pd) and rhodium (Rh). Of these metals, rhodium is the most effective for reducing the oxides of nitrogen to nitrogen gas. Generally in the automotive catalyst field, rhodium is highly dispersed on aluminum oxide (Al.sub.2 O.sub.3) supports and is active as the metal, designated Rh.sub.x.sup.(0) where the subscript x refers to the number of Rh atoms in a Rh crystallite or assembly. It is known that active Rh.sub.x.sup.(0) may be converted to an oxidized species Rh.sup. (I) by a complex oxidation process in which surface hydroxyl groups present on the oxide support phase are consumed in the presence of carbon monoxide (CO) gas. Both the carbon monoxide gas and the surface hydroxyl groups simultaneously participate in the destruction of the Rh.sub.x.sup.(0). This process can be reversed using hydrogen gas (H.sub.2) which reduces the Rh.sup.(I) surface species, present as Rh.sup.(I) (CO).sub.2 species, to Rh.sub.x.sup.(0) while regenerating the isolated hydroxyl groups.
A number of patents disclose processes for hydrogenation and reduction reactions which are effected in the presence of catalysts. It would be generally expected by those persons skilled in the art that the silation reactant that is reacted with the metal catalyst will react with reduced metal sites, such as for example Rh.sub.x.sup.(O).
U.S. Pat. No. 3,336,239 discloses that metal compounds containing a metal component in one of its oxidation states, such as rhodium Rh(.sup.+1+2+3), can be made extremely active for use as hydrogenation and/or reduction catalysts by reacting the metal compounds with a silane containing Si-H bonds. This patent uses silanes to reduce oxidized metals to make catalysts by breaking the Si-H bond. In the present application there are no Si-H bonds. This patent states that the starting silicon compounds can constitute a monomer, a polymer, or any mixture thereof, such that silanes having both halo or organo substituents may be used to reduce oxidized metals to make catalysts.
U.S. Pat. No. 3,389,092 discloses a process for the preparation of silica-alumina catalysts or supports for catalytic metals by reacting a solid adsorptive hydroxylcontaining alumina with an organo silane. This is then followed by heating and oxidation to make a silica-alumina powder.
U.S. Pat. No. 3,956,179 discloses a process for chemically modifying the surfaces of inorganic solid bodies having hydroxyl groups suitable for use as stationary phases in chromatography and in catalysts. With reference to chromatographic separation processes, this patent teaches that alkyl chains of alkylhalosilanes are chemically bound through a Si atom to the oxygen atom of the hydroxyl group in a reaction which substitutes the hydroxyl hydrogen with the alkyl silane by forming --O-Si-alkyl linkages. Further, as to liquid and gas chromatography this patent states that the Si-alkyl chains carry reactive hetero atoms which can then be substituted by desired groups.
U.S. Pat. No. 4,005,046 discloses a catalyst carrier substance which may be obtained by reacting an inorganic particulate solid having surface hydroxyl groups with an organo silicon compound containing sulphur. Further, this patent states that the carrier may be converted to a supported catalyst by a reaction with a compound or complex of platinum or rhodium.
In spite of these prior art disclosures, there remains a very real and substantial need for a process for the chemical modification of an aluminum oxide supported rhodium catalyst while leaving the reduced rhodium sites, Rh.sub.x.sup.(0) essentially unreacted. This process may be employed to create an automotive catalyst converter which is beneficial for environmental protection. More specifically, there is a need for such a chemical modification using silation wherein surface hydroxyl groups are removed from the aluminum oxide support so that the conversion of active rhodium Rh.sub.x.sup.(0) to oxidized rhodium species Rh.sup.(I) by reduction of the active hydroxyl groups is substantially resisted or prevented.